A typical cholesteric liquid crystal display device comprises a layer of cholesteric liquid crystal material disposed between a front transparent plate and a back plate. Electrodes formed of a transparent, conductive film an applied to the inner surfaces of the plates. In response to an electric field applied between the electrodes perpendicular to the front plate, V⊥, the cholesteric liquid crystal material switches between a transparent state and a reflective state. In the reflective state, the cholesteric material intercepts light through the front plate and reflects the light back through the front plate. A typical display comprises a plurality of pixels, each corresponding to a distinct region of cholesteric liquid crystal layer having electrodes that switch the region' independent of other regions. In the reflective state, the region forms a bright pixel that cooperates with light from surrounding pixels to create an image for the display.
A preferred type of cholesteric liquid crystal material is composed of chiral dopants and a nematic host. The nematic molecules are elongated along a longitudinal molecular axis. In the presence of the chiral dopant, the molecules form a helical arrangement in the reflective state, with the molecular axes perpendicular to the helical axis and the helical axis perpendicular to the front plate. The helical arrangement is characterized by a pitch which is directly related to the wavelength, λ, of the reflected light. In contrast, when the molecules are not aligned, referred to as a scattered state, the material becomes transparent. Application of an electric field along the helical axis is effective to switch the cholesteric liquid crystals between the transparent state and the reflective state by scattering the molecules or causing them to align. Once switched, the cholesteric material remains in the state even after the electrical field is removed, until an appropriate field is applied to cause the material to switch again.
The wavelength of reflected light is determined by the pitch of the cholesteric molecules in the helical arrangement. This, in turn, is determined by the proportion of the chiral dopant to nematic molecules. Different levels of chiral dopants result in different pitch lengths. For common materials, small amounts of dopant result in reflected light having longer wavelengths, corresponding to red color, whereas higher concentrations of dopant are effective to reflect light having shorter wavelengths, shifted towards green or blue color.
Thus, one drawback of conventional cholesteric displays is that the material, once formulated, is limited to reflecting light of a predetermined wavelength, that is, a single color. In order to get a multicolored display, more that one cholesteric liquid crystal material is required. For example, layers of red, green and blue cholesteric liquid crystal materials may be stacked and then selectively switched between reflective and transparent states to form different colors. However, such stacks tend to be thick, heavy, and expensive to manufacture. They also generally require separate drivers to control the switching of each color layer.
Accordingly, there has been a long felt need for a cholesteric liquid crystal device that utilizes a single layer of liquid crystal material to form a multicolor display and that may readily manufactured as a thin, relatively inexpensive device.